Microwave filters



Jan. 14, 1958 M. ARDn-l Erm.

MICROWAVE FILTERS 3 Sheets-Sheet l Filed May 8, 1952 5@ @lai [ef-4* $9 @4 @f vl+z m mmm JW a... Y RSA. E m/N Npu R .EMA o VRaK n mw ma A o l .E q G n W. d

C M IM w m lm. m m W 4 E 5 m Jan.- 14, 1 958 M. ARDmv ET Al. MICROWAVE FILTERS Filed May v8., 1952 3 Sheets-Sheet 2 5 P @wwf Y Omar/E TRUE N NA .L R 55.2% o WKSK H :WMC` 7A Amm ME e United States Patente() u' MICROWAVE JFILTERS Maurice Arditi, Clifton, N. J., and Georges A. Deschamps, New York, and Jack Elefant, Brooklyn, N. Y., assignors to International Telephone-and Telegraph Corporation, a corporation .of Maryland kApplication'May 8, 1952, Serial No.- 286",761 5 Claims.- Cl. V333,-73)

This inventionv relates to microwave.transmissionsystems and morev particularly tomicrowavelters specially applicable tok microwave;r printed. transmission ylines ,and circuitry.

In the copending applications of D. D. Grieg vand F. Engelmann, Serial No, 234,503, ledJuneSO, 1951;, and M. Arditi and P., Parzen, Serial No. 286,764, lledl May 8, 1952, now Patents 2,721,312 and 2,774,046, Vrespectively, a type of microwave vtransmission :line is disclosed comprising, in one of its` simplest forms, two conductors printed or otherwise disposed in .substantially vv.parallel relation on opposite sides of a. strip or layer .of .dielectric material a small fraction of ,a quarter wavelength thick. The two conductors may be of the same width o r one may be made wider than the other. The dielectric betweeny the two conductors may be `of substantially the same width as the narrowestof theA two conductors or wider accordingto the relationships desired. 'Y

One of-the objects of this invention jis to7 provide a microwave filter of narrow bandwidth characteristics utilizing a section ofthe aforementioned parallel strip type of line. l j

Another object of the invention. is to provide amicrowave filter of narrow bandwidthcharacteristics Vwhi :l1.i s small, light in weight, and, relatively simple'and inexpensive to make. -V

One of the features of this invention is the manner of providing spaced susceptances of large value to define a resonant section or cavity in. a parallel ribbon-like conductor form of line. The susceptances, broadly speaking, may comprise the placing of two obstacles or other discontinuity structnres in the-line at spaced -points'to form a resonant cavity section therebetween. tinuities may' comprise any structure capable of .producing susceptances of desired values; For example-"such a susceptance may be in the form of conductor projections disposed part Way across the space between the parallel 2,820,206 v`Iaaltented Jan. 14, 1958 ICC 2 Fig. 2.18.@transactional riewnakenalpas linee# 0f Fig. 1; Fis .Sis .a Circle diagram use@ .in explaining susceptance characteristics of thelpost obstacles used in the filter; yFig. A is afrequency response curveforilters, made in 1acfordance with thev principles ofthe present invention;

Fig.f5 is a `fragmentary.View `ofalilteri showingpne resonant cavity section and a capacitive ,stirew tnningar Irangement therefor; 'd

Fig. 6 is alongitndinal sectional,mevr/,taken alongfline '6J'6of.Fig-s;

`-Figs.'7, 8,.and9 kshow in plan view.modiied arrangements n for tuning resonant cavity sections 'that` may be substitntedfor the screw tuningfarrangement' ofligs. Sand@ s` Fig. 1 0 isa plan view of. anothenformotllilter. made in accordancewith the principles `ofglthis invention;` 1

n Fig. 11 isa longitudinal sectional viewftakenwalong These discon- .1,

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ribbon conductor, and a shunt` susceptance maycomprise one or more vconductors extending entirely-across the space between the parallel conductors; Theseconductor projections or interconnecting conductorsmaybea`r ranged in many diiferent ways, including various conductor shapes, sizes, dierent locations relative to the vlongitudinal axis of the line and in. manyf-dilerentplural arrangements. Another feature of the y:invention v-is the method and means forV tuningV the resonant spacing.'L As morev fully described :hereinaftenwthis Aituning 'ofthe resonant spacing or cavity may'A be Iaccomplished inyarious Vways,` including .Vernier capacitive-screws,

`line compressors or stretcher-s, andlateral lineprojections or protnberances.

c l lThe abovefmentioned andvother features. and `objects of .this invention and the mannerof attainingthem will bewherein;

Fig..1 is a plan view of oneformg of tilteigirr` accondance with the principles of f this invention;

line 11-,11 of Fig. 1o; E, ligs. 12, 13, and 14 are fragmentary views in plan of ,filter sections. showing differentr arrangements shunt susceptances; 5 M Fig. `15 is a cross-sectional view taken along-line 1 5T15 QiFig 14;

fEig, 16 is a fragmentary view in, plan of.another. sus ceptanearrangement fortheliltensection; figs. 17 and 18 aresectiOnaI-vieWstaken\,along,.1ines .17,-17and18-f-18-of Fig. 16; and N l Fig. ,1 9 isaview in planlshowing-twoirectly(coupled 4resonant,sections of sti-1l -another',l.,ernh c idiirrent10Q-the invention. j ,leferringto Figs. 1y and 2, the microwave transmission lineshown isof the printed circuit ,typeconiprisigfafirst or line. c onductor 1 anda second or baseffcondiictor z with alayer of dielectric material therebetween.: The conductive material .may be applied and/or'shapedor `,The microwave lineof Figs. 1 and 2 isv shownprovided 'with spaced obstacles in theform of shnntsnsceptances as indicated by the posts 4, 5; `6,. 7; 8, 9., The tliree Acavity vsections provided by theseposts` are iin dicatedtls being of lengthsv 1, the adjacent seetions fbeir 1gcoupled by quarter wavelengthsections of line.. .lWe haVe found that a post 0.052" in diameter disposed in interconnecting relation across the line conductors `1, z atfthe' longitudinal' axis of the line, isequivalent approximatelyto a shunt susceptance of, -6.5. Larger yvalnes4 ofvsusceptance can be obtainedwith larger diameter; posts,l Aninerease in the size of post, however, is ,ac:o rn'panied withacoro responding increase in loss.

Als,o, a platey of. condnctive material or other obstruction mayfbe, placed onor made integral with one or the other of the Iii-ley conductors, eitherl open Vor' shorted at the ends,.with respect yto, the other'line conductor. For further dios'closnre:| on'f'this form of susceptance arrangements, referenceY may be tiled May8, 195 With-these large values ofsusceptances, a micro-wave .filtert` is obtained by proper spacing of the susceptances, :and for:multiplecavity resonatorssu-ch filter.r sections may Vb ef quai'ter wave coupled ordirectlpconpl'edt Thedis- `-t-allee for spacing: thesusceptances :.schaszobtai'ned by 3 the posts 4 and 5, for example, may be obtained from the circle diagram shown in Fig. 3 and from the knowledge that in such lines the ratio, ,phase velocity/velocity of light, is practically constant with frequency, that is, the line is not dispersive.

The two circles and 11 shown on the Smith admittance chart, Fig. 3, were made from tests of single and double post susceptances of the forms indicated in the drawing. It will be observed that the single post at 10a is twice the diameter of each post employed at 11a, yet the circle 10 is smaller than circle 11` thus indicating that the loss of the single post while small is greater than that of the double posts. Relevant valuesobtained from the charts in these two tests are shown inthe drawing.

By way of example, a resonant section oflin'e utilizing two posts to form the resonant section centered at 4700 mc. per second were placed at a distance of 14.5 mm.

measured between the axes of the posts. The frequency response of such a cavity'is shown by curve 13Fig. 4. Where a multiple section quarter wave coupled, such as illustrated in'Fig. 1 is provided, the frequency response curve is narrowed as indicated by curve 14, thereby insuring a narrow band pass ilter. As shown by the circle diagram of Fig. 3, the insertion loss of a single cavity at center frequency was about 1.4 db in a line having 3 db/meter losses. This value of1.4 db was obtained from a sample using liberglass as the dielectric having Y large dielectric losses. These insertion losses, however, can be obtained by using Teflon dielectric. lt will also be observed that from the circle diagram, the distance between the posts 4 and 5 is rather critical and that an error of 0.05mm. in distance between 'axes of the posts will shift the centerfrequency of the pass lband about 160 megacycles.

Since thel spacing of the posts is critical, it is desirable to produce as accurate' as' possible the location ofthe posts. location the exact or nearly exact location may be obtained by the following method. The hole for the post is made slightly larger than the post or wire conductor to be used. A drop of solder is applied to both ends of the post, one to connect the'post to the conductor 2 and the other to conductor 1. Such solder locations are indicated at l5 and 16, Fig. 2. The solder is maintained soft by means of a soldering iron so that the position of the post may be adjusted slightly longitudinally or laterally with respect to the longitudinal axis of the line until an optimum position is obtained, whereupon the solder is permitted to freeze. This method of testing for the position of the post may be made by using any suitable measuring technique, one satisfactory method of making such measurements being disclosed in the copending application of G. A. Deschamps, Serial No. 333,164, tiled January 26, 1952. When the location of the post is determined by experimentation, it may be reproduced accurately by photographic methods and printed circuit techniques.

lt is recognized, however, that some slight variation is always possible since a slight variation in position may alter the susceptanze value or the effective spacing cf the cavity section. It is accordingly desirable to have some additional means for tuning a resonant section after fabrication of the filter. In Figs. 5 and 6 one form of tuning arrangement is shown. This form includes a tuning screw 17 which preferably is provided on the longitudinal axis of the line, although it need notbe, for adjustment relative the spacing between conductors 1 and 2. Further, the screw 17 may be located anywhere desired between the two obstacles. Thedielectric 3 is provided with an opening 18 for this purpose. Where the After an approximate determination of the-post conductor 1 or 2 through which the screw extends is of such small thickness that it would not provide satisfactory threaded connection for the screw, 'a threaded ywasher 19 is applied thereto for reception of the screw.

cned 'by increasing the width of the line and shortened by narrowing the width of the line. This lengthening and shortening ofthe linev has reference to line wavelength. In Fig. 8 a similar tuning effect is obtained by placing a piece of wire 22 crosswise of the conductor 1 between the posts 4 and 5. Tuning is etected by applying a drop of solder 23 to the wire 22 and while maintaining the Ysolder soft with a soldering iron, the wire is nudged from one position to'another until an optimum tuning is obtained at which point the solder is allowed to freeze. Once the exact location of the piece of wire 4is obtained, the same may be reproduced by printing lugs on the conductor '1 as indicated at 24 and 25 in Fig. 9. Should further tuning be required, the ends of the lugs 'may be sliced away as indicated by line 26 until optimum tuning is obtained. If too much is sliced away, additional conductive material may be added to the ends of lthe lugs by soldering.

Referring to Figs. l0 and 11, another lter embodiment is shown wherein each resonant section is defined by four posts27,"28,r 29, and 30. As shown, two such resonant sections are indicated at 31 and 32 which are quarterwavecoupled. The line section incorporating the postsis shown to comprise conductors 33 and 34 supported in spaced relation. at. the ends thereof with bodies of dielectric material 35 and 36. This permits the provision of free space withinA the length of the tilter. This air space, however, requires a different spacing between theconductors 33 and 34 for impedance match with re- 'spect to the line having solid dielectric between conduc- Fig." 11,.;where e isV the dielectric constant of the solid dielectric.y If additional support is desired, a foam polyethylene or foam Teflon having nearly unity dielectric constant and very low losses may be used as the spacing material between conductors 33 and 34. While the conductors 33 and 34 are shown -to be of unequal width, they may of course be equal in width if desired. To provide additional tuning for the resonant sections and also for the quarter wave coupling, capacity screws 37 are provided as indicated. This form of tilter section where two small posts are .used such as 27 and 2S in the place of a single large post such as post 4, Fig. 1, to form one of the susceptance points in the line, is of advantage in that the smaller posts provide the equivalent susceptancey of the one larger post and at the same time present less total loss. While the filter of Figs. 10 and 1l is shown to have air dielectric between the conductors 33 and 34, it should be understood that the dielectric material 35 and 36 could be continued throughout the length 4This staggering relationship provides for a delay in phase which may be of advantage in certain installations for suppressing undesired modes.V Another arrangement for suppressing multi-modes is shown in Fig. 13 wherein the lter susceptances are formed by providing a row of small posts as indicated at 4Z and 43. Where a series of such posts are used, smaller wires may be employed than would be required for a single post for the same susceptance. Such arrangements of posts may be employed independently where the suppression of multimoding is required. It will also be recognized by those skilled in the art that a dispersion of posts along the line either in groups or following a given dispersion pattern may be used for producing a lattice effect.

Referring to Figs. 14 to 18, different obstacle arrangements to provide desired susceptances at points along the line are shown to indicate a few of the possible different arrangements and shapes and dispositions that may be employed in following the principles of this invention. In Figs. 14 and 15, for example, a filter section is delined by four obstacles 44, 45, 46, and 47 in the form of vanes. These vanes may be of selected widths to simulate iris sections of desired dimensions. The iris sections represented by vanes 44 and 45 provide shunt susceptance. the vanes being embedded in the dielectric 48. In Figs. 16, 17, and 18, the conductor projections are shown in the form of irses, which if desired may extend the full width of the conductors 49 and 50 as indicated by the iris portions 51, 52, Fig. 17. The irises 51, 52 need not be the full width of the conductors depending on the amount of susceptance required. The tuning arrangement indicated at 53, Fig. 16, and shown in more detail in Fig. 18, comprises adjustable irises 54 and 55, one for each of the conductors 49 and 50. The two irises 54 and 55 are centrally located in the resonant section and are opposed in spaced relation one to the other.

Referring to Fig. 19, a directly coupled filter is shown comprising resonant sections 56 and 57 formed by three series of small posts 58, 59, and 60. Each resonant section is preferably provided with an adjustable capacity screw 61. While the series of posts as indicated at 58 comprise three small wires shunted across the line conductors, such series could be replaced by one or two posts or even by a greater number than three posts, if desired.

While we have described above the principles of our invention in connection with specific apparatus, it is to be clearly understood that this description is made by way of example only and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.

We claim:

1. A microwave lter comprising a pair of ribbon-like strip conductors, one of said conductors being of a width narrower than the other whereby the mode of propagation of wave energy therealong approximates the TEM mode, means disposing said conductors with the wide surfaces thereof in spaced substantially parallel relation a fraction of a quarter wavelength apart to provide a waveguide having the space between said conductors conductively open along the sides thereof, said means disposing said conductors in spaced substantially parallel relation includes bodies of dielectric material terminating in tapered portions and spaced apart longitudinally of said strip conductors, said strip conductors between said dielectric bodies being spaced a shorter distance apart than at the location of said bodies, the spacing changing gradually adjacent the tapered portions of said bodies, and a pair of susceptive obstacle discontinuities disposed in longitudinally spaced relation relative said conductors and in the space between said conductors to form a resonant section therebetween, the narrow spacing of said conductors and the different widths thereof operating to minimize radiation effects laterally through said open sides in the vicinity of said obstacle discontinuities.

2. A microwave lter comprising a pair of ribbonlike strip conductors, one of said conductors being of a width narrower than the other whereby the mode of propagation of wave energy therealong approximates the TEM mode, means disposing said conductors with the wide surfaces thereof in spaced substantially parallel relation a fraction of a quarter wavelength apart to provide a waveguide having the space between said conductors conductively open along the sides thereof, said means disposing said conductors in spaced substantially parallel relation includes bodies of dielectric material spaced apart longitudinally of said strip conductors, said strip conductors between said dielectric bodies being spaced a shorter distance apart than at the location of said bodies, the spacing changing gradually adjacent the terminated portions of said bodies, and a pair of susceptive obstacle discontinuities disposed in longitudinally spaced relation relative said conductors and in the space between said conductors to form a resonant section therebetween, the narrow spacing of said conductors and the different widths thereof operating to minimize radiation effects laterally through said open sides in the vicinity of said obstacle discontinuities.

3. A microwave filter according to claim 2, wherein the lilter section between adjacent obstacle discontinuities is provided with a conductive screw carried by one of said line conductors for adjustment relative to the other of said line conductors.

4. A microwave transmission line according to claim 2, wherein said discontinuities each includes a conductor post disposed in conductive relation with respect to one of said line conductors and extend into the space between said line conductors.

5. A microwave transmission line according to claim 2, wherein at least certain of said discontinuities include conductors interconnecting said line conductors.

References Cited in the file of this patent UNITED STATES PATENTS 2,406,945 Fell Sept. 3, 1946 2,411,555 Rogers Nov. 26, 1946 2,424,982 Higgins Aug. 5, 1947 2,540,488 Mumford Feb. 6, 1951 2,579,324 Kock Dec. 8, 1951 2,710,946 Grieg et al. June 14, 1955 2,721,312 Grieg et al. Oct. 18, 1955 2,774,046 Arditi et al Dec. l1, 1956 FOREIGN PATENTS 584,153 Great Britain Ian. 8, 1947 655,803 Great Britain Aug. 1, 1951 

